1. Field of the Invention
The present invention relates to a phase-change drive of a high transfer rate, and more particularly, to a phase-change optical disc drive with a semiconductor laser power control and a method of writing/reading a phase-change optical disc at a high transfer rate by controlling the laser power of the semiconductor laser.
2. Description of Related Art
Typical phase-change optical discs in which information can be rewritten include a so-called DVD-RAM. DVD-RAM adopts an EFM (Eight-to-Fourteen Modulation) method for signal modulation to drive a semiconductor laser to emit multi-pulses for generation of recording waveforms for marks of 3T to 11T.
FIG. 1 is a timing chart of recording waveforms in DVD-RAM. As shown, a mark 3T is written with a single light pulse while a mark 11T is recorded with nine light pulses. Each light pulse is immediately followed by a bias power 2 set to a lower value than a bias power 1 corresponding to an erasure power to control the crystallization speed of the phase-changed recording medium. Further, both leading and trailing pulses have a period of about 1T, and they are generated with predetermined time delays (T.sub.SFP and T.sub.SLP in FIG. 1), respectively, from the clock pulse.
DVD-RAM uses a clock frequency of 29 MHz (user transfer rate of 11 Mbps). Both the leading and trailing pulses have a width of about 35 ns. The recording power of these light pulses is detected by a laser power monitoring detector, and then sampled and given a predetermined value, thus the recording pulse is controlled to always have a constant power. For example, the peak power is controlled by sampling the light power of the leading or trailing pulse, and the bias power 1 is controlled by sampling the light output of the multi-pulse. Note that the bias power 2 is controlled by sampling a reproducing output detected by an RF signal detector.
For a phase-change optical disc drive of a higher transfer rate than the normal one of DVD-RAM, however, it is necessary to increase the clock frequency. For a clock frequency of 100 MHz (use transfer rate of 38 Mbps), for example, the leading and trailing pulses will have a width of less than 10 ns, so it will be difficult to accurately sample the light output for controlling the recording power. Furthermore, wen the light pulse is less than 10 ns in width, it will easily be affected by noise component and also a sufficient band of the optical system cannot be assured, so that the recording power cannot be controlled.